Globular clusters in M104: Tracing kinematics and metallicities from the centre to the halo

TL;DR

Fahrion et al. present a comprehensive GC catalogue for M104 by combining literature data with new MUSE, FLAMES, and OSIRIS spectroscopy, enabling a coherent analysis of GC kinematics and metallicities from the galaxy center to the halo. They apply full-spectrum fitting with pPXF and SSP libraries (E-MILES/XSL/Coelho) to derive line-of-sight velocities and metallicities across hundreds of GCs, achieving a dispersion-dominated system with low overall rotation. The GC metallicity distribution is broad and mildly metal-poor with metal-rich GCs present at all radii, signaling a complex assembly history that includes in-situ metal-rich formation and accretion of metal-poor systems. Their results, including a dynamical mass estimate within 1.8 $R_{eff, GCS}$ and a nuanced red/blue GC dichotomy in kinematics, support a rich formation history for M104, with implications for the structure of its metal-rich spheroid and halo components.

Abstract

As ancient star clusters, globular clusters (GCs) are regarded as powerful tracers of galaxy evolution and assembly. Due to their brightness and compact sizes, GCs are employed to probe the kinematics and stellar population properties of galaxies, from the central regions out into the halo where the underlying stellar light becomes too faint for spectroscopic studies. In this work, we present a comprehensive study of the GC system of M 104 (NGC 4594, also known as the Sombrero galaxy) based on literature spectroscopic catalogues and newly collected data from Very Large Telescope (VLT) MUSE integral-field spectroscopy combined with multi-object spectroscopy from VLT FLAMES and OSIRIS at the Gran Telescopio de Canarias (GTC). We present a new catalogue of 499 GCs with radial velocity measurements that span from the inner disc region out to $\sim$ 70 kpc (24$^{\prime}$). In addition to velocities, we measure metallicities from the MUSE, OSIRIS, and FLAMES spectra of 190 GCs. Together with literature values, we collected a sample of 278 metallicities. Comparing GCs observed with multiple instruments, we find a good agreement of velocity and metallicity measurements. Studying GC kinematics with a simple model confirms a decreasing velocity dispersion profile and low rotation velocities. The blue GCs appear to be more dispersion-dominated, while the red GCs follow the kinematics of the stars more closely. We find a large scatter of GC metallicities with distance from the centre and metal-rich GCs are found over all radii. We discuss that the GC metallicity distribution with a broad metal-poor component likely reflects the complex assembly history of M 104.

Figures (14)

• Figure 1: Footprints of the different datasets on a greyscale DSS2 image of M 104. MUSE pointings are shown in cyan squares, each with a field of view of $1\arcmin \times 1\arcmin$. The red circles indicate the field of view of the FLAMES GIRAFFE instrument, with a diameter of 25, and the orange rectangles show the field of view of OSIRIS, which spans $7.5\arcmin \times 6\arcmin$. The full greyscale image is 40$\times$ 40 (110 kpc $\times$ 110 kpc).
• Figure 2: Spectra of GC #276 for which spectra from MUSE (top), OSIRIS (middle), and FLAMES (bottom) are available. The black lines show the data, and the blue and red lines are the fits with pPXF. Shaded regions were excluded due to sky line residuals. The MUSE and OSIRIS spectra were fitted with XSL SSPs, while we used high resolution stellar models from Coelho2014 to fit the FLAMES spectrum shown here.
• Figure 3: Distribution of GCs extracted from MUSE data. Individual GCs are marked with dots colour-coded according to their LOS velocity. Small dots are GC candidates without match to the HST catalogue from Spitler2006 and Harris2010. The greyscale image shows the white light image from MUSE.
• Figure 4: Outlier removal based on GC velocities and projected distances. Top: Line-of-sight velocities as a function of projected galactocentric distance for the full sample of 721 unique sources. Different colours refer to different samples. In case a source is found in multiple samples, only the velocity with the smallest uncertainty is shown. Bottom: Difference between $m_N$ and $m_{N-1}$ as a function of GC number $N$ (left, see Eq. \ref{['eq:mn']}) and the derivative (right). The vertical grey line indicates our chosen cut-off $N_\text{cut}$ = 222, right after the last big jump as seen in the derivative. From this value, the corresponding $v_\text{max}(R)$ curve was derived, which is shown as the grey dashed line in the top panel. The shaded regions show the curves when assuming $N_\text{cut} \pm 5$.
• Figure 5: Spatial distribution of GC velocities (top) and metallicities (bottom). The large panels on the left show the full distribution of velocities and metallicities, respectively. Circles refer to this work; triangles refer to GCs listed in the Dowell2014 and AlvesBrito2011. Black crosses show dwarf galaxy candidates near M 104 from Crosby2024. The smaller panels on the right side show the velocities and metallicities in different bins, as indicated in the panel titles. Fig. \ref{['fig:GC_vel_metal_sky_alt']} shows a zoom-in of the central 5$\times$ 5.